1. Field of the Invention
This invention relates to spiral inductors and more particularly to a structure of, and a method for forming, spiral inductors on a semiconductor substrate. Spiral inductors relate to analog high frequency electronic circuits within, but not limited to, receiver and transmitter portions of wireless telecommunications devices such as cellular telephones, pagers, and Personal Communications Services (PCS) products.
2. Description of the Related Art
There are various geometries and structures existing in the art relevant to spiral inductors. These structures, however, have shortcomings in their electrical performance, cost, and/or physical size.
One type of conventional spiral inductor, illustrated in FIG. 1, is commonly implemented using an integrated circuit (IC) process over an insulating, semi-insulating, or semi-conducting substrate, having an optional ground plane.
For the design of balanced circuits, however, two inductor elements are needed, as illustrated in FIG. 2, items L1 and L2. Therefore, using the conventional art, two separate FIG. 1 inductors are needed, which consumes significant physical area. Further, because of a potential mismatch in inductance, the conventional separate inductors have potential problems with circuit balance. Still further, this conventional arrangement lowers the operating frequency range of the inductors, since one terminal of each inductor is at AC ground potential.
Other methods of implementing inductors are described by Jamison et al., "Inductively Coupled Push-Pull Amplifiers For Low Cost Monolithic Microwave ICs", 1982 GaAs Symposium, (Jamison '82), and Selmi, et al., "Design of an X-Band Transformer-Coupled Amplifier Circuit With Improved Stability and Control", IEEE Journal Of Solid State Circuits, Vol. 28, No. 6, June 1993, (Selmi '93).
Jamison '82 and Selmi '93, however, are inter-wound primary-secondary coils of coupling transformers and do not comprise inter-wound halves of a center-tapped inductor. This can be seen referring to Jamison '82 where the FIG. 1 schematic shows a left primary and a right secondary coil, each appearing center-tapped in a schematic sense because of their common point of electrical connection. However, looking to FIG. 2 of Jamison 82, the schematic's depicted center-tap of both the left inductor pair and the right inductor pair are physically the connection of two separated independent inductors. The interwound traces comprise the top half of the left (primary) pair interwound with the top half of the right (secondary) pair. Selmi '93 shows a similar arrangement of inter-wound primary and secondary traces. Hence, what is depicted as a center tapped inductor in both Jamison '82 and Selmi '93 consumes separate, and hence substantial, area for the two halves.